1. Field of the invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine for controlling an air-fuel ratio in a gas mixture supplied to the internal combustion engine.
2. Discussion of background
There have been proposed various types of fuel control apparatus. A conventional fuel control apparatus as described in Japanese Unexamined Patent Publication No. 212643/1985 will be described. FIG. 16 is a diagram showing a conventional fuel control apparatus.
In FIG. 16, a numeral 1 designates an air cleaner, a numeral 2 an air-flow meter for measuring an intake air quantity, a numeral 3 a throttle valve, a numeral 4 an intake air manifold, a numeral 5 a cylinder, a numeral 6 a water temperature sensor for detecting a temperature of cooling water, a numeral 7 a crank angle sensor, a numeral 8 an exhaust air manifold, a numeral 9 an exhaust gas sensor for detecting the concentration of a component (for instance, oxygen concentration) in exhaust gas, a numeral 10 a fuel injection valve, a numeral 11 an ignition plug, and a numeral 12 a control apparatus.
A crank angle sensor outputs a reference position pulse for each reference position of crank angle (each 180.degree. for a four cylinder engine and each 120.degree. for a six cylinder engine) and a unit angle pulse for each unit angle (for instance, each 1.degree. ).
A crank angle can be detected by counting the number of unit angle pulses after a reference position pulse has been read by the control apparatus 12. The number of revolution of engine is also detected by measuring a frequency or a period of the unit angle pulses.
In the example of FIG. 16, the crank angle sensor 7 is disposed in a distributor.
A control apparatus 12 is constituted by a micro computer comprising, for instance, a CPU, an RAM, an ROM and an input output interface. The control apparatus 12 receives an intake air quantity signal S1 from the air-flow meter 2, a water temperature signal S2 from the water temperature sensor 6, a crank angle signal S3 from the crank angle sensor 7, an exhaust gas signal S4 from the exhaust gas sensor 9, a battery voltage signal and a throttle full-opening signal and so on, and operates the signals to thereby calculate a value of fuel injection quantity to be supplied to the engine, whereby a fuel injection signal S5 is outputted. A fuel injection valve 10 is actuated by the signal S5 to thereby supply a predetermined amount of fuel to the engine.
Calculation of a fuel injection quantity T.sub.i is conducted in the control apparatus 12 by using the following equation: EQU T.sub.i =T.sub.p .times.(1+F.sub.t +KMR/100).times..beta.+T.sub.s( 1)
where T.sub.p is a basic injection quantity which is obtained by a formula T.sub.p =K.times.Q/N where Q is an intake air flow rate, N is an engine revolution number and K is a constant, F.sub.t is a correction coefficient corresponding to a cooling water temperature for the engine which assumes a greater value as the water temperature decreases, and KMR is a correction coefficient of a heavy load (for instance, it is memorized in a data table as a value corresponding to both the basic injection quantity T.sub.p and the engine revolution number N, the coefficient being readable from the data table, T.sub.s is a correction coefficient changing dependent on a battery voltage, which is to correct variation in voltage to actuate the fuel injection valve 10, and .beta. is a correction coefficient corresponding to the exhaust gas signal S4 from the exhaust air sensor 9, by which a feed-back control of the air-fuel ratio of a gas mixture can be effect so that the air-fuel ratio is maintained at a predetermined value, i.e. at or near a theoretical air-fuel ratio of 14.6.
Since the air-fuel ratio of the gas mixture is controlled at a constant level by the feed-back control, correction by the cooling water temperature and correction by the heavy load coefficient become meaningless. Accordingly, the feed-back control by the exhaust gas signal S4 can only be conducted when the correction coefficient F.sub.t of the water temperature and the correction coefficient KMR of the heavy load are zero. FIG. 18 shows the relation between the items of correction and sensors.
The conventional fuel control apparatus is so adapted that while a feed-back control is carried out in response to the signal of the exhaust gas sensor, correction under a heavy load condition is made in accordance with a basic fuel injection quantity T.sub.p and an engine revolution speed N, i.e. an intake air quantity Q and an engine revolution speed N. In other words, the correction is made by an open loop control. Accordingly, there is a possibility that an air-fuel ratio is deviated from the optimum air-fuel ratio (the optimum air-fuel ratio is to obtain the greatest torque, which has usually a valve of 13 or around which is different from a value for a feed-back control of air-fuel ratio) due to scattering of the air-flow meter or the fuel injection valve, and deterioration of them with time. When, such phenomenon takes place, a stable operation of engine can not be obtained.
Further, since the air-flow meter measures a quantity of air staying in the intake air pipe as well as an amount of air sucked into the engine, a value obtained by the feed-back control of air-fuel ratio does not indicate the true value.